The measurement of time-resolved wall shear stress is an important aspect for both fundamental scientific studies and applied aerodynamic applications. In general, wall shear stress is the friction between a moving fluid and adjacent surface. Wall shear can also be referred to as skin friction, which can be used to determine drag, transition, and flow separation. In engine applications, wall shear stress can lower efficiency and increase fuel consumption. In particular, quantitative measurement of wall shear stress has received considerable attention in development of aerospace vehicles. Applications for wall shear stress measurement include feedback sensors for flow control. However, despite several research efforts, time-accurate, continuous, direct measurement of fluctuating shear stress has been elusive due to stringent spatial and temporal resolution requirements.
Currently, MEMS sensors exist for both direct and indirect shear stress measurement. Indirect sensors rely on a correlation between a measured flow property and the shear stress. Previously reported micromachined indirect shear stress sensors include thermal sensors, micro-fences, micro-pillars, and laser based sensors. In contrast, direct sensors measure the integrated shear force on a sensing area such as a floating-element structure. Micromachined direct sensors in the past have used capacitive, optical, and piezoresistive, transduction schemes. Each sensor design demonstrated progress towards the development of shear stress sensors. However, the prior sensors have shown limited performance in terms of sensitivity drift, and insufficient dynamic range, bandwidth, and/or minimum detectable signal (MDS).